1. Field of the Invention
This invention relates to implanted medical devices and, more particularly relates to a physiological waveform morphology discriminator and method for use in a pacemaker for characterizing the origin of cardiac depolarizations.
2. Description of the Prior Art
A variety of disease mechanisms may result in cardiac dysrhythmias. Typically these dysrhythmias are characterized by electrical instability in the cardiac tissue which results in abnormal mechanical activity of the heart. The abnormal mechanical activity results in the reduction of the rate at which oxygenated blood is circulated throughout the body. The parameter is called cardiac output. If the loss of cardiac output results from a heartbeat slower than a normal heartbeat responding to the same physiologic demand, the dysrhythmia is called a bradycardia. In contrast, an abnormally rapid beating of the heart which also results in reduced cardiac output is generically termed a pathologic tachycardia.
Pacers for the treatment of bradycardia are known from U.S. Pat. No. 3,478,746 to Greatbatch. This patent teaches a demand type (VVI) pacemaker which provides a stimulus to cardiac tissue through a pacing lead; if no naturally occurring cardiac activity is sensed within a preset time period referred to as the escape interval. Consequently, the stimulating pulses are supplied to the heart only when the intrinsic heart rate drops below a preset minimum corresponding to this escape interval.
The intrinsic heart rate is detected by a demand pacer through a sense amplifier. A sense amplifier typically provides passband amplification at the frequencies which predominate in the intracardiac signal as obtained from the left ventricle. The amplification stage is typically followed by a level detection stage which provides a logic level output if the input physiological waveform meets both passband and amplitude criteria. Examples of sense amplifiers as used in demand pacers include the device taught by U.S. Pat. No. 3,927,677 to Gobeli.
More recent pacers have been proposed for the treatment of tachycardia. Both U.S. Pat. Nos. 3,698,398 and 3,693,627 to Berkovits teach the use of a pacer therapy to treat a tachyarrhythmia. Like a demand pacer, the tachy treatment pacer utilizes a sense amplifier to detect the cardiac depolarizations. Typically the tachyarrhythmia treatment therapy is invoked when the rate of cardiac depolarizations as measured by the sense amplifier reach a preset limit between 120 beats per minute and 200 beats per minute. There are two problems associated with the use of a simple rate detection system for the detection of a pathologic tachyarrhythmia. One problem is that pathologic tachyarrhythmias can have rates within the range shared by normal heart activity. There is, in effect, an overlap between the physiologically normal heart rate which does not require treatment and the pathologic heart rate which does require therapy.
An additional problem relates to the detection by the pacer system of beats of ectopic or abnormal origin. A normal heat beating at a physiologically normal rate may be interrupted by occasional premature ventricular contractions (PVC). These PVC's appear on the EKG interspersed among sinus beats in the patient's electrocardiogram. Consequently, a rate detection algorithm operating on sensed depolarizations detected by a sense amplifier could interpret a PVC as a tachyarrhythmia.
The PVC as well as other beats of abnormal origin differ in their shape or morphology from a normally conducted sinus beat. Typically, for example, the amplitudes and durations associated with PVC's are larger than those associated with normal beats and the slope or rise time of the signal is somewhat less than the intrinsic deflection of a normally conducted R-wave. These time domain differences in the signal do not produce substantial changes in the frequency spectra of the signal. As a consequence, conventional sense amplifiers relying on frequency domain parameters such as bandpass amplification are incapable of distinguishing normally conducted R-waves from abnormal beats despite the differences in their time domain morphology.
The ability to distinguish between ectopic beats and sinus beats would aid immeasurably in the formulation of pacer therapies for cardiac dysrhythmias in general, and tachyarrhythmias in particular.